7.6
The Cahn Ingold Prelog
R-S Notational System
Two requirements for a system
for specifying absolute configuration
1.
need rules for ranking substituents at
stereogenic center in order of decreasing
precedence
2.
need convention for orienting molecule so
that order of appearance of substituents
can be compared with rank
The system that is used was devised by
R. S. Cahn, Sir Christopher Ingold, and
V. Prelog.
The Cahn-Ingold-Prelog Rules
(Table 7.1)
1. Rank the substituents at the stereogenic
center according to same rules used in
E-Z notation.
2. Orient the molecule so that lowest-ranked
substituent points away from you.
Example
1
1
3
4
4
3
2
Order of decreasing rank:
4>3>2 >1
2
The Cahn-Ingold-Prelog Rules
(Table 7.1)
•
•
•
1. Rank the substituents at the stereogenic
center according to same rules used in
E-Z notation.
2. Orient the molecule so that lowest-ranked
substituent points away from you.
3. If the order of decreasing precedence traces
a clockwise path, the absolute configuration
is R. If the path is anticlockwise, the
configuration is S.
Example
1
1
3
4
4
3
2
2
Order of decreasing rank:
432
clockwise
R
anticlockwise
S
Enantiomers of 2-butanol
H
H
CH2CH3
CH3CH2
C
OH
H3C
(S)-2-Butanol
HO
C
CH3
(R)-2-Butanol
Very important! Two different compounds with
the same sign of rotation need not have the same
configuration.
Verify this statement by doing Problem 7.7 on
page 269. All four compounds have positive
rotations. What are their configurations
according to the Cahn-Ingold-Prelog rules?
Stereogenic center in a ring
H3C
H
R
H
H
—CH2C=C > —CH2CH2 > —CH3 > —H
7.7
Fischer Projections
• Purpose of Fischer projections is to show
configuration at stereogenic center without
necessity of drawing wedges and dashes or
using models.
Rules for Fischer projections
H
Cl
Br
F
Arrange the molecule so that horizontal bonds
at stereogenic center point toward you and
vertical bonds point away from you.
Rules for Fischer projections
H
Br
Cl
F
Projection of molecule on page is a cross.
When represented this way it is understood
that horizontal bonds project outward, vertical
bonds are back.
Rules for Fischer projections
H
Br
Cl
F
Projection of molecule on page is a cross.
When represented this way it is understood
that horizontal bonds project outward, vertical
bonds are back.
7.8
Physical Properties of
Enantiomers
Physical properties of enantiomers
Same:
melting point, boiling point, density, etc
Different:
properties that depend on shape of molecule
(biological-physiological properties) can be
different
Odor
CH3
O
H3C
CH3
O
CH2
(–)-Carvone
spearmint oil
H3C
CH2
(+)-Carvone
caraway seed oil
Chiral drugs
Ibuprofen is chiral, but normally sold as
a racemic mixture. The S enantiomer
is the one responsible for its analgesic
and antiinflammatory properties.
H3C
H
CH2CH(CH3)2
C
HO
C
O